PROJECT SUMMARY Paclitaxel is a standard chemotherapy for Triple Negative Breast Cancer (TNBC) that targets dividing cells. While being effective for many patients with this disease, adverse effects as well as development of resistance underscore the importance of identifying rational drug combinations that can be used with taxanes to reduce their dosing while enhancing their efficacy. Studies in this proposal focus on using a novel class of drugs known as BET inhibitors (BETi) to disrupt proper mitosis/cytokinesis and potentiate paclitaxel sensitivity. BETi control gene expression and have minimal toxicity in mice. We have found that this new class of drugs induce polyploidy and negatively impact the way breast cancer cells divide. In the first aim of this proposal, we will determine if BET inhibitors impair TNBC cell growth by blocking their ability to accurately divide their chromosomes when producing daughter cells. We will also identify the spectrum of mitosis/cytokinesis- associated genes that are directly controlled by BETi. Thus far, our studies have revealed that BETi suppress the expression of a large group of related proteins known as mitotic kinesins. Notably, many individual mitotic kinesins can cause paclitaxel resistance, making it essential to block the levels of the extended family of these proteins. Other than our findings with BET inhibitors, no other drugs have been identified that globally reduce the levels of these kinesins. Since taxanes primarily kill breast cancer cells by disrupting the splitting of chromosomes during cell division, and upregulated expression of mitotic kinesins can induce resistance to these drugs, we hypothesize that BETi will potentiate paclitaxel response. We will test this possibility in cell lines and mouse models in Aim 2. Lastly, we have found that BETi suppress the expression of FOXM1, a factor that is essential for mitosis that also regulates mitotic kinesin expression and can induce paclitaxel resistance. In the third aim, we will determine if the FOXM1 gene is a direct target of BETi that mediates their effects on mitosis/cytokinesis, kinesin expression, and response to taxanes. If so, this would suggest that changes in FOXM1 levels in tumors treated with BET inhibitors may be a useful biomarker for predicting tumor response to the drug in future clinical trials. Together, our proposed studies will determine how the new class of drugs known as BET inhibitors suppress the growth of breast cancer cells and if these drugs can potentiate the effectiveness of a standard of care drug, paclitaxel, in one of the most highly aggressive forms of breast cancer. These studies are the first to discover a mitotic mechanism of action of BETi in TNBC as well as identify a rational therapeutic approach for their movement into patient care.